Super critical fluid chromatography allows to separate a component, i.e. an extractant from another component, i.e. a matrix, by making use of a super critical fluid as the extracting solvent. By means of SFC and HPLC, various substances can be chemically analyzed, identified and quantified. Making use of carbon dioxide as a super critical fluid in SFC applications, the extraction of the substances has to be conducted under super critical conditions. Regarding carbon dioxide as the super critical fluid of choice, the extraction has to be conducted above the critical temperature of 31° C. and above a critical pressure of 74 bar.
For keeping CO2 or a CO2-mixture in a liquid state inside a chromatography column, the entire chromatography system has to be kept on a predefined pressure level. For this purpose, downstream of the chromatography column and downstream of a respective detector, a back-pressure regulator is typically provided, to keep the pressure inside the chromatography system on a predefined level. Upstream of the chromatography column there is typically provided a preparation stage comprising at least one pump in order to pressurize the at least one solvent.
With HPLC or SFC chromatography applications the solvents, such like ethanol and/or CO2 have to be pressurized up to a level of several hundred bar, typically up to 400 bar, as high as 1000 bar or even above. In these pressure ranges solvents like CO2 typically exhibit a comparatively large compressibility. When making use of conventional pumps, such like reciprocating pumps, feed flow pulsations are typically observable in the fluid flow at a pump's outlet.
In FIG. 2 a conventional pump 20 in form of a duplex pump with two pump heads 22, 24 is schematically illustrated. In a chromatography environment, such like in an SFC system, the inlet 26 of the pump 20 is connected to a cooler 12 which is in fluid communication with a storage reservoir 102 being at least partially filled with a solvent, e.g. liquid CO2. By means of the cooler 12 the liquid solvent can be pre-cooled to a well-defined process temperature. The reciprocating pump 20 with its two pump heads 22, 24 is further equipped with numerous check valves 23, 25 downstream and upstream of the pump heads 22, 24 which are arranged in parallel between the inlet 26 and the pump's outlet 21.
In operation, each pump head 22, 24 makes use of a piston which is driven by a cam that alternately aspirates fluid from the inlet 26 by increasing the available pump head volume, then dispenses the fluid to the outlet 21 by decreasing this volume. The flow direction is controlled by the check valves 23, 25 that isolate the pump heads 22, 24 from the output pressure during aspiration and from the input pressure during dispensing. Even though FIG. 2 illustrates a duplex type arrangement of a pump 20 other geometries, hence a simplex pump with only one pump head or triplex or quad pumps with three and four pump heads are also generally applicable for chromatography applications.
The pulsations of the fluid flow over time arising at the output 21 of the pump 20 are schematically illustrated in FIG. 3. As can be seen from the diagram 60 the flow rate and hence a flow volume of the pressurized liquid over time at the outlet 21 of the pump 20 may drop down to about 5% of the maximum flow rate as illustrated by various flow rate drops 61 of the diagram according to FIG. 3. These inevitable variations or pulsations of the fluid flow limit the precision of the chromatography system.
In document U.S. Pat. No. 8,215,922 B2 this problem is identified. There, a pressurized pumping system is described that comprises a first pump that operates to increase the pressure of a fluid and that comprises a second pump, connected in series to the first pump, wherein the second pump receives the pressurized fluid from the first pump and meters the fluid to an output of the second pump. An input pressure of the fluid that is received by the second pump from the first pump is held near to or slightly below the output pressure of the fluid at the output of the second pump such that a minimal density change occurs to the fluid traveling between the input and output of the second pump.
Connecting two pumps in series requires that both pumps have to provide a comparatively large flow volume. Moreover, with this approach it is in particular the pumping speed of a booster pump that needs to be controlled, which may be rather costly and elaborate.